Sequential programming, threads, causality, and the nature of time

by Martin Fitzpatrick Last updated PyQt5 FAQ
Heads up! You've already completed this tutorial.

Dan wrote

I just started with PyQt and it seems pretty cool but tried to use a "progress bar" to track a long I/O process which introduced me to threads. I have things mostly working and understand how a long I/O would mess up the event queue for functions called AFTER the I/O process is called but I have experienced the following:

  1. FIRST I try & update some GUI widgets
  2. Immediately AFTERWARDS I call the long I/O function

Somehow step #2 interferes with step #1! It's like a time machine! ;-) I'm guessing that the call to step #1 just places GUI commands in a queue which immediately gets interfered with by step #2? So.... I guess all I'm asking for is a way to "flush the > queue" or somehow force step #1 to complete before step #2? I tried calling various "update" & "repaint" functions but that doesn't seem to help. I can (of course) deal with this via threads but it seems a bit inelegant... Any suggestions?

amohgyebigodwin10691

Qt always runs application logic before it runs UI code, even if Ui code was called first, that is what is going on.

To fix this call the I/O function in a separate thread.

PedanticHacker

Over 10,000 developers have bought Create GUI Applications with Python & Qt!
Create GUI Applications with Python & Qt5
Take a look

Downloadable ebook (PDF, ePub) & Complete Source code

Also available from Leanpub and Amazon Paperback

[[ discount.discount_pc ]]% OFF for the next [[ discount.duration ]] [[discount.description ]] with the code [[ discount.coupon_code ]]

Purchasing Power Parity

Developers in [[ country ]] get [[ discount.discount_pc ]]% OFF on all books & courses with code [[ discount.coupon_code ]]

Hello Dan, welcome to the forum! 🙂

Martin has written a wonderful article about threads and thread-blocking in GUI applications. You'll find all your answers there.

Dan

Thanks! It really is an excellent article.

Well done, you've finished this tutorial! Mark As Complete
[[ user.completed.length ]] completed [[ user.streak+1 ]] day streak

Sequential programming, threads, causality, and the nature of time was written by Martin Fitzpatrick .

Martin Fitzpatrick has been developing Python/Qt apps for 8 years. Building desktop applications to make data-analysis tools more user-friendly, Python was the obvious choice. Starting with Tk, later moving to wxWidgets and finally adopting PyQt.

Sequential programming, threads, causality, and the nature of time was published in faq on (updated ) . Feedback & Corrections welcome in our public issue tracker.

 pyqt5 pyqt python qt qt5